1. Field of the invention
The present invention relates to a process for separating nitrogen gas from a gaseous mixture principally comprising nitrogen and oxygen, such as air and, more specifically, to a process for obtaining a high-purity nitrogen gas by pressure-swing adsorption (hereinafter referred to as "PSA") process using adsorbers packed with carbon molecular sieve (hereinafter referred to as "CMS").
2. Description of the prior art
CMS, with its different adsorption rates for various gases, has the excellent property of removing by adsorption oxygen, carbon dioxide, moisture and the like in a short period of time, thereby directly separating nitrogen.
Separation of nitrogen from a gaseous mixture such as air by PSA process with an adsorbent of CMS generally comprises the steps of pressurization adsorption, pressure equalization, desorption and regeneration and the like and utilizes a plurality of adsorbers to change over these steps alternately and successively, thereby continuously obtaining nitrogen. Pressure equalization is a process which comprises transferring the gas remaining in an adsorber having completed adsorption, by utilizing pressure difference, to another adsorber having completed regeneration, until the internal pressures of the two adsorbers become nearly the same. This process is carried out to decrease the unit requirements of raw material gas and power.
Various measures are taken, in the course of each of the above steps, to increase the purity of product nitrogen gas. For example, a method comprises cleaning an adsorber being regenerated by passing therethrough a small amount of product nitrogen; one comprises, when an adsorber has just completed both regeneration and pressure equalization and is ready for adsorption operation, flowing back product nitrogen in an amount required for transferring low-purity nitrogen staying at a space near the outlet of the adsorber during the equalization operation, to the bed in the adsorber; one comprises discharging out of the system or returning, for re-use, into raw gas, the nitrogen with relatively low purity that has generated just after the start of adsorption operation; and the like.
Pressure equalization causes sudden pressure changes in the two adsorbers concerned, whereby CMS tends to flow and be pulverized due to grinding action among granules. To minimize such pulverization, there is proposed reduction in the flow rate of transferring gas to prolong depressurization time.
PSA-type nitrogen gas separation apparatuses with adsorbing beds comprising CMS and using a raw material gas of air or the like, are based on simple mechanism and can produce on-site, when necessary, inexpensive nitrogen. The apparatuses have the feature of reliable safety and capability for conducting full-automatic unmanned operation, and have hence expanded their market rapidly.
When compared with the existing processes for producing nitrogen, as represented by cryogenic separation, the nitrogen obtained through PSA-type nitrogen separating apparatuses has lower purity, which has been constituting the largest obstacle to spreading uses of the gas. Where the purity standard of nitrogen (purity of nitrogen is the total volume percentage of nitrogen and argon; hereinafter the same will apply) is set at at least 99.9%, the product yield decreases and the unit requirement of raw gas worsens markedly. To meet requirement for high-purity nitrogen, it is therefore necessary to add to a PSA apparatus a purification apparatus utilizing a process which comprises adding hydrogen to the nitrogen separated by PSA, permitting it to react with remaining oxygen in the presence of a catalyst to form water and removing the water, or the like. Employment of this process or the like creates many problems such as use of dangerous hydrogen, contamination of unreacted hydrogen into product nitrogen and increase of equipment cost.